Current commercial offerings of metal-pigmented inks cannot be successfully used in a proofing application except as transferred via thermal transfer. This is because metallic pigments are produced as flakes which have a large aspect, their length to width, may be tens, or even hundreds, of microns in size. This makes it very difficult to pass the metal flake through the narrow orifice of a typical inkjet head. Similarly, toner based digital proofing systems rely on electrostatic or electro-dynamic forces to hold the toner particles on the image drum. However, metal flakes are conductive and thus bleed the charges off, allowing the toner to fall off of the image drum before printing can occur. There have been no commercial solutions known to the inventors that have addressed these problems.
On the other hand, thermal transfer uses a plastic donor sheet on which a clear resin coating with dispersed pigment has been cast. The print heads contain heating elements that raise the temperature of the coating until it becomes fluid and then is transferred by contact with the receiving sheet. These thermal transfer films are produced in much the same way as paint films and the metallic flakes are dispersed into the polymeric resin before being cast onto the plastic sheets. The thermal transfer method is very slow and very expensive (proofs cost near $100.00 each for A3 format) and the imaging systems for applying the donor to a substrate can also be rather expensive (over $10,000). Such a system can be, for example, a proofing printer such as the Kodak Approval NX and XP, shown at, for example, http://graphics.kodak.com/US/en/product/proofing/kodak_approval/default.htm?_requestid=5068.
Moreover, when using such thermal transfer technology, the level of brightness must be developed on the donor roll. This requires development and warehousing of multiple donor rolls with varying metallic pigments. For example, paint coatings have five or six levels. Converters that produce brochures for the automotive industry generally offer three levels, bright, medium and dull—giving appearances from that of the shiny front side of a sheet of foil down to an unpolished metal sheet.
What is needed in the art is an inexpensive method to vary the appearance of metallic inks for use in proofing applications that can be easily iteratively repeated, so as to fully match a desired package or other substrate design.